The solar beam charge is called Synthesis btw.
Anyway, Synthesis's hitbox is extremely useful for many reasons.
1: It's Ivysaur's best Out Of Sheild or Crouch Canceled get out of my face move under pressure.
2: At lower damage percents, the move can be used following a short hop fair or bair as a combo move. Think of Synthesis in combos kinda like how marth short hop double fairs, except ivy short hop fairs or bairs and then synthesis.
3: Synthesis immediately cancels when it hits the ground, which lets you follow up hitting someone with it extremely fast, allowing for easy up smashes, grabs, vine whip, or solar beam, or just more combos.
4: Though this more of a pet thing I do, I like to B-reverse it a lot after dash jumps for fake outs and surprise attacks.
5: The move comes out extremely fast.
6: It heals you, and pushes you a step closer to your best finisher, solar beam.